An electrophotographic copying apparatus having a duplex copying function is known which is useful for saving the number of the copy sheets to be consumed and the space required for the preservation of the duplicate documents. In a copying apparatus of this type, images are printed on one face of a copy sheet during a first cycle of copying operation, followed by a second cycle of copying operation by which images are further printed on the opposite face of the copy sheet. During the latter cycle of copying operation, the copy sheet bearing the printed images on one face thereof is turned upside down before the sheet is brought into contact with the photosensitive drum. In order to automate such a sheet reversing function, a known copying apparatus having a duplex function has an intermediate copy-sheet recovery stage for temporarily storing the copy sheet on which images have been printed during the former cycle of copying operation. The latter cycle of copying operation for the reverse face of the copy sheet is thus started with the copy sheet supplied from such an intermediate copy-sheet recovery stage. An extra duplex copy-sheet supply tray is thus provided at the intermediate copy-sheet recovery stage for receiving the copy sheet bearing the printed images on one face and thereafter supplying the copy sheet for the second cycle of copying operation. Such a duplex copysheet supply tray may be detachably assembled to the copying apparatus when it is desired to perform a duplex mode of copying operation and is selected from among some trays which are respectively allocated to predetermined sizes of copy sheets.
In a copying apparatus of this type, it may happen that the duplex copy-sheet supply tray fails to properly conform to the size of the copy sheet transported to the intermediate copy-sheet recovery stage. When this occurs, the copy sheet transported to the intermediate copy-sheet recovery stage can not be correctly received by the duplex copy-sheet supply tray and might cause jamming that will put an end to the intended duplex copying operation abortively or will lead to a more serious failure of the apparatus.
In a copying apparatus having an intermediate copy-sheet recovery stage, it is thus important to confirm and assure that the duplex copy-sheet supply tray currently assembled to the apparatus properly conform to the size of the copy sheet to be supplied from the main copy-sheet supply stage of the apparatus. The operator of the apparatus is for this reason required to make such a confirmation each time he desires to perform a duplex mode of copying operation.
One approach to providing a solution to this problem may be to prohibit the apparatus from carrying out a duplex mode of operation while producing any warning signal when it is found that there is a discrepancy between the duplex copy-sheet supply tray in use and the size of the copy sheet selected to be used. This approach is useful for the prevention of the jamming but still requires the operator to take any time-consuming actions on the control panel to remove such a discrepancy.